FIELD OF INVENTION
This invention concerns a method for processing milk in order to obtain cheese.
The main patent has for its object a method which makes it possible to obtain, starting from milk, on the one hand a liquid filtration product containing the major part of the soluble constituents of the initial milk, such as lactose, mineral salts, non-protein nitrogen substances and, on the other, a liquid containing all the casein of the initial milk, the greater part of the soluble proteins and a small part of the soluble constituents, at concentrations substantially equal to those existing in cheese prepared from milk at the end of the whey drainage process. Thus, a notable object of the invention is a method making it possible to prepare from milk, such as secreted by the milch-animal, a liquid raw material having the concentration and composition of a drained cheese.
The essential characteristic of the invention described in the main pateht is, therefore, a method for processing milk and certain dairy by-products by ultrafiltration in order to obtain a product having substantially the same composition as a cheese, but in the liquid form, the said product then being converted into cheese.
The main pateht also provides for the processing of all types of milk or dairy by-products previously used in cheese manufacture. It is possible, for example, to start with cows' goats' or ewes' milk, which may be subjected in a known manner to moderate pre-heating.
In practice, the method described in the main patent consists in contacting milk at a pressure of from 1 to 50 kg/cm.sup.2 with at least one semi-permeable membrane the average pore diameter of which lies in the range of 3 to 30 m .mu., thus providing a first liquid, or filtration liquid passing through the said membrane, and that a second liquid or retentate is obtained which does not pass through the said membrane and having a higher concentration than the starting product, notably in proteins. The pore diameter of the semi-permeable membrane and pressure are selected so that lactose and mineral salts in the aqueous phase of the milk pass through the semi-permeable membrane in substantially the proportions they appear in the aqueous phase of the milk. Generally speaking, the retentate has a concentration in proteins, lactose, mineral salts and total solids content substantially equal to that known to exist in cheese made by renneting of a milk.
In summary, the milk product to be processed in contact with a semi-permeable membrane is placed under pressure and typically a turbulent flow and/or a laminar flow is imparted to the liquid milk product in contact with the membrane. Both types of flow agitate the liquids adjacent to the membrane and permit the obtaining of a higher content of solids in the retained liquid retentate.
The pressure applied to the milk product in contact with the membrane can be applied with a pump and/or with an inert gas under pressure, such as nitrogen.
Various types of membranes were used to prepare the retentate. Membranes available on the market are made either from polyolefines (as is the case for membranes sold under the trade name "Diaflo" by the firm Amicon, U.S.A.), or from polyacrylonitrile or polyvinylchloride (as is the case for membranes sold by the firm Rhone-Poulenc, France), or from cellulose acetate (as is the case for membranes sold by the firms De Danske Sukkerfabriker, Denmark and Abcor, U.S.A.).
The dairy product which does not pass through the membrane, or retentate, becomes more protein-enriched as the process keeps going on.
Thus, according to the main patent, the previous step may optionally be repeated (as by recycling) starting, in this case, with the said second liquid, until a liquid product is obtained which does not pass through the membrane, and the concentration of which, notably in proteins, is substantially equal to that of the cheese type food product desired.
The present invention constitutes a development of the method of the main patent in as far as the forms of conservation of the products obtained after processing the milk by ultrafiltration through the membrane are concerned.
A notable object of the invention is to allow on the one hand, said products to be conserved (kept) for a long period of time and, on the other, to facilitate their transport over long distances; even to countries which do not have raw materials suitable for cheese making and converted into cheeses in such countries.
It is known that the high water content in food products can, to a certain extent, be considered as being a nuisance on the keeping quality and the preservation of the nutritional value of products, as this high water content facilitates the development of microorganisms, chemical deterioration (autoxidations for example), the breakdown through enzymes action, among others.
Said high water content also constitutes a drawback on the level of the cost of transport of food products, as it is necessary to transport large amounts of water in proportion to the amounts of useful dry matter transported.
Therefore, at all times efforts have been made to dehydrate food products in order to store them and facilitate their transport. Spray-drying and freeze-drying are the techniques most frequently used nowadays for dehydrating food products. In the field of dairy products, whole or skimmed milk is usually dehydrated by the first mentioned technique to produce a powdered milk.
It is also known that, although it is possible for the powdered milk so obtained to regain its original concentration by adding a suitable amount of water, the conversion of said reconstituted milk into cheese is subject to a certain number of difficulties as far as coagulation with rennet (soft coagulum which is difficult to handle, and drainage of the coagulum obtained). Furthermore, such a conversion always provides cheese having organoleptic qualities which are inferior to those of cheese obtained by the conversion of the initial milk.
It is further known that it is not possible to envisage the dehydration of most types of cheese by spray-drying, as this leads to a complete destruction of the shape and texture of the cheese.
Finally, it is known that although dehydration of various types of cheese by freeze-drying is theoretically possible, its cost is prohibitive owing, first, to the thickness of said cheeses, a thickness which considerably lengthens the time required to obtain complete dehydration, and secondly, to the many difficulties which arise when rehydration is effected, or even the impossibility of such rehydration.